390 PROFESSOR E. RAY LANKESTER. 



which I made at) that time have not hitherto been published, 

 though used by me in illustration of lectures. Recently these 

 drawings have been made into woodcuts for illustration of a 

 work on the Lancelet, with which I am busy, and I am indebted 

 to the publishers (Messrs. Kegan Paul and Co.) for the oppor- 

 tunity of submitting these drawings on the present occasion. 

 The fact of a segmentation of the musculature of the tail of 

 Appendicularise into myotomes has, I believe, been quite recently 

 recorded by Langerhans, but I am unable to refer more exactly 

 to his observation. A similar segmentation of the musculature 

 seems to occur in some Ascidian tadpoles, though I am not aware 

 that it has been described in detail. 



The metamerisation of the musculature in the tail of Fritillaria 

 furcata, shown in the woodcut Fig. 2 a (where /(• points to one of 

 the myomeres), is not obvious in the living or even in a recently 

 killed individual. The division between the separate groups of 

 muscular fibres only became obvious after the specimens had 

 been for some time mounted in glycerine, having been first 

 treated with picric acid as they lay upon the object-slide. I 

 could not trace a distinct fibrous septum separating the myo- 

 meres from one another, but merely a break in the continuity of 

 the muscular fibres. It is probable that a very delicate mem- 

 brane separates each myomere from its successor, but my speci- 

 mens did not enable me to distinguish such. 



As is well known, the nerves which pass off from the nervous 

 axis of the tail do not proceed from ganglionic masses of nerve- 

 cells situated on that structure, but simply from the fibrous 

 nerve-cord, only the first pair having a ganglionic structure at 

 their point of origin. Hence the repetition of paired " spinal '* 

 nerves in the tail, though corresponding to the seven myomeres 

 or muscular segments, has less significance as an indication of 

 metamerism than would be the case were there a repetition of 

 nerve-cell-masses in each segment. 



The metamerism of the tail of Fritillaria, though it may be 

 viewed as an incipient formation of vertebral segments, appears 

 to be most satisfactorily explained as a remnant of a more fully 

 expressed " vertebration,"' which was possessed by a larger and 

 more elaborate ancestor of the Appendicularise, of which existing 

 forms are the reduced and degenerate descendants. The exceed- 

 ingly small number of cell-units which build up the structure of 

 existing Appendicularise, and the whole history of the develop- 

 ment of the Ascidians, seems to favour this supposition. 



The remaining structures shown in the diagrammatic wood- 

 cuts illustrating these remarks are explained in the list of re- 

 ferences. 



